Nearly a decade ago (wow!) I suggested an alternate way to handle undead's energy drain (and swords of life stealing etc.) would be to let the PCs keep their level, and all the spells/day, chances to hit, saves, skills that come with it. They just lose the hit die and associated hit points that go with it.
I never did implement that idea.
Now, I've got another idea that is also worth considering. Energy drain doesn't sap levels, but it does sap XP. So a 6th level Fighter that is energy drained drops from however much XP they had to 24,000xp (half way from 5th to 6th) but keeps the hit points, fighting ability, and saves of a 6th level character. It just takes them a lot longer to level up to 7th. Getting level drained again would drop them to 12,000xp, halfway between 4th and 5th. And if they get down to halfway between 1st and 2nd and get energy drained again, they die and rise the next night as undead themselves.
The penalty is pretty big, but retaining all the level abilities would allow them to keep adventuring with their companions. Also, with my rule of "you keep your level when you die and roll up a new PC" it would work better.
Tavern Update - Forthcoming MIA and Summer Plans
6 hours ago